Struts 2 + JSON + AJAX pdf downloading issue - json

Question

I have a script in which on Click on link i need to download PDF file present in temp folder on server and this i am doing it by calling AJAX function. that AJAX function is calling struts 2 action method which is reading content of the file and write it to output stream.
and now problem is if that file is not present in temp folder. that is throwing exception. so i decided to handle it by using json. if that file exist in temp folder. i am mapping the key to
jsonObject.put("exist", true)
and if that file does not exist in temp folder. i am mapping the key to
jsonObject.put("exist", false)
and in struts.xml i am handling the action by with this way
<action name="DisplayStaticPdf"
class="com.ui.DisplayStaticPdfAction">
<result name="success" type="json"/>
</action>
and in script i am handling it through this way
function downloadDoc(actionName){
$.ajax({
url: actionName,
type: 'post',
dataType: 'json',
error: function(data) {
if(data.exist!= null && data.exist == 'false'){
alert('The document you are trying to download does not exist at location. Please make sure file exist .');
}
},
success: function() {
}
});
}
and when file is not present, it is showing me the same alert. but when file is not present, it does nothing and does not download the pdf present in temp folder.
my action class looks like ->
String pdfFileName = null;
BufferedInputStream bis = null;
BufferedOutputStream bos = null;
String rawFileName = "abc.pdf";
String returnString = "fileNotExist";
jsonObject = new JSONObject();
ServletOutputStream out = response.getOutputStream();
response.setContentType("application/pdf");
URL url = new URL(fileURL);
URLConnection conn = url.openConnection();
if (conn.getContentLength() != 0 && conn.getContentLength() != -1)
{
bis = new BufferedInputStream(conn.getInputStream());
bos = new BufferedOutputStream(out);
try
{
byte[] buff = new byte[2048];
int bytesRead;
while (-1 != (bytesRead = bis.read(buff, 0, buff.length)))
{
bos.write(buff, 0, bytesRead);
}
jsonObject.put("exist", true);
}
returnString = SUCCESS;
}
else
{
jsonObject.put("exist", false);
returnString ="fileNotExist";
}
return returnString;
hope you guys understand my issue. Anybody please put light on this whether i am doing it in right approach. or is there is any other easy way to do it
Thanks,
Ankit